The contribution of quartz and the role of aluminum for understanding the AAR with greywacke

Precambrian Greywacke from Lower Lusatia (Germany) has been well known as an alkali sensitive aggregate for several years. It can cause considerable damages in concrete buildings due to an Alkali–Aggregate-Reaction. The investigations are focused on quartz as the main releaser of silica, its characterization and its behavior in an alkaline solution. But there are no relations between quartz properties and the alkali sensitivities of greywacke samples. To understand this fact the role of aluminum which greywacke releases in different amounts into the alkaline solution too must be considered. Aluminum affects the silica concentration by three different mechanisms. The result is always a decrease of the silica concentration in the solution caused by an alumino silicate formation. The silica bound by alumino silicate structures can be quantified by 27Al–NMR-spectroscopy. The expansions of concrete samples can now be described much well as a function of a so called “free” silica. Based on this results a direct test method for the assessment of the alkali sensitivity of greywackes could be created and suggestions for an inhibition can be given.